But for some reason, in some howler monkeys, the melanin switches to pheomelanin, which produces yellow, red or orange tones instead.

“These observations represent totally anomalous cases of pigmentation, as these have never been reported in captivity or in the wild before,” the authorsconclude.

“We are not aware of similar changes in pigmentation in other primates or other groups of mammals.”

This marks the first time scientists have noticed such a rapid change in the pigmentation of primate fur, or any other mammal for that matter.

And while it’s still not clear exactly why these pigments are changing, pesticides are suspected to be the main culprit.

It turns out, that the pigment responsible for the monkey’s yellow fur contains sulfur, and sulfur, incidentally, forms the basis for most pesticides used in the world.

The researchers propose that as these Costa Rican monkeys are exposed to more pesticides, the abundance of sulfur messes with the pigment of their fur, changing the structure of melanin and, therefore, their overall colour.

This isn’t some crazy hunch, either. In Costa Rica, pineapple, banana and African palm oil farms have recently begun to use a greater number of sulfur-containing pesticides.

And already scientists are noticing that most animals with unusual colours come from the forests surrounding these farms.

Black howler monkeys may just be the latest victims of these chemicals. In Costa Rica, these monkeys feed mainly on leaves from trees surrounding these sorts of farms.

As such, they probably consume a significant amount of pesticides, which could have the power to influence the resulting melanin.

The idea is certainly interesting, but right now it’s just speculation. The researchers say that more research needs to be done to find out the underlying cause of this pigment change.

And it’s important that we do just that. All of that bright yellow fur may look harmless, but the conspicuous colour change could make these monkeys more vulnerable to attack from jaguars and other predators.

After all, the poor souls with completely yellow fur stand our like ripe fruit against the dense green forest.